1. Field of the Invention
The present invention relates to an apparatus for detecting variation abnormality in air-fuel ratio among cylinders of a multicylinder internal combustion engine, and in particular, to an apparatus suitably applicable to an internal combustion engine with a plurality of banks.
2. Description of the Related Art
In general, an internal combustion engine with an exhaust purification system utilizing a catalyst efficiently removes harmful exhaust components using the catalyst, and thus needs to control the mixing ratio between air and fuel in an air-fuel mixture combusted in the engine. To control the air-fuel ratio, an air-fuel ratio sensor is provided in an exhaust passage in the engine to perform feedback control to make the detected air-fuel ratio equal to a predetermined air-fuel ratio.
On the other hand, a multicylinder internal combustion engine normally controls the air-fuel ratio using the same control variables for all cylinders. Thus, even when the air-fuel ratio control is taken place, the actual air-fuel ratio may vary among the cylinders. In this case, a small variation can be absorbed by the air-fuel ratio feedback control, and the catalyst also serves to remove harmful exhaust components. Consequently, such a small variation is prevented from affecting exhaust gas emission and from posing no obvious problem.
However, if, for example, fuel injection systems for some cylinders fail to significantly vary the air-fuel ratio among cylinders, the exhaust gas emission disadvantageously deteriorates. Such a significant variation in air-fuel ratio, as it degrades the exhaust gas emission, is desirably detected as an abnormality. In particular, for automotive internal combustion engines, there has been a demand to detect variation abnormality in air-fuel ratio among the cylinders in a vehicle-mounted state (what is called OBD: On-Board Diagnostics) in order to prevent a vehicle with deteriorated exhaust gas emission from travelling. Attempts have recently been made to legally obligate the on-board abnormally detection.
For example, an apparatus described in Japanese Patent Laid-Open No. 2010-112244, upon determining that any cylinder has an abnormal air-fuel ratio, identifies the abnormal cylinder by decrementing the duration of injection of fuel into each cylinder by a predetermined value until the cylinder with an abnormal air-fuel ratio misfires.
In the meantime, when a variation in air-fuel ratio among the cylinders is detected based on the angular velocity of a crank shaft, rotation of a timing rotor fixed to the crank shaft is detected by a crank angle sensor. However, a product variation among timing rotors may vary the position, in a rotating direction, of each of a large number of projections formed on a circumferential surface of the timing rotor.
In view of the above-described circumstances, it is an object of the present invention to suppress a detection error caused by a product variation among timing rotors, improving detection accuracy.